First off, embrace the nerves. I pitched at the highest level & freely admit that I was borderline terrified prior to taking the mound. I was able to find a way to channel that into adrenaline & focus once the hitter actually stepped in. This is not a negative (fear, apprehension, etc) Explain that it is OK & to use the extra juice.
Visualization: People talk about being "in the zone." This is how you get there. Practice, repetition, work, visualize the nasty slider on the black & it will be so.
A novice gets the sign & thinks like this: "Slider. Ok, let's see what happens."
A Pro gets the sign & thinks like this: " Slider. Ok, like it. 1-2 count, just busted him in with fastball. Start this middle down & he has no chance.